A common technique for manufacturing PCBs employs surface mount technology, where components are secured to conductive pads on the surface of the circuit board. Conductive paths in or on the circuit board interconnect the pads. Components are affixed to these pads by conductive mounting materials, referred to hereinafter as mounting materials. These mounting materials are compounds with both adhesive and electrical conduction properties, such as solder paste and conductive epoxy. Solder paste is a viscous paste that includes finely ground solder particles in a flux base. Conductive epoxies are organic compounds which have adhesive properties sufficient to secure electrical components to circuit boards and which are electrically conductive. A variety of conductive epoxies are available, for example, from Epoxy Technology, Inc., Billerica, Mass.
In one known practice, solder paste is printed onto a circuit board through an apertured mask placed in selected registration over the circuit board. A material-applying sweep by a squeegee-like blade across the mask applies the solder paste to the circuit board in a pattern reflective of the apertures in the mask. Generally, the application of solder paste, or other mounting material, to a circuit board occurs with a single sweep of the blade. The use of multiple sweeps, or multiple blades, is commonly deemed undesirable. After each sweep of the blade, the processed circuit board is removed and replaced with a fresh board. After applying solder paste to the circuit board, leads of various electronic components are placed on the PCB at the sites bearing the solder paste. The solder paste is then heated and allowed to cool, thereby securing the components to the board and connecting them electrically to the conductive runs in the board.
A considerable problem in PCB production relates to the fidelity of applying the mounting material. Fidelity, as used herein, includes resolution, reproducibility and definition. Herein, resolution refers to the minimum distance between two adjacent mask apertures capable of ensuring no inadvertent contact between mounting material sites on the circuit board. Reproducibility refers to the maximum variation in mounting material patterns between any two PCBs produced with the same mask. Definition refers to the precision with which the mounting material pattern reflects the aperture pattern of the mask. Common fidelity problems include, for example, contamination of the mask bottom with mounting material and void spaces in the mounting material deposited on the circuit boards. Void spaces in the deposited mounting material become increasingly pronounced with decreasing size of the mask apertures. Fidelity problems are of significant concern with PCBs designed to accommodate high densities of components; minute errors in mounting material placement can render these boards useless. Fidelity limitations presently determine the maximum allowable density of PCB components. Increasing the fidelity of mounting material application will allow for the low cost, mass production of PCBs with higher densities of components than is presently possible.
Attempts to increase the fidelity of the mounting material application process include increasing the pressure exerted by the blade upon the mask, using slow blade sweep rates, and using angular blade sweeps across the mask. Each of these techniques suffers from disadvantages. Increased blade pressure decreases mask life, increases blade wear, and scavenges mounting material from mask apertures. Scavenging of mounting material occurs when the blade, during the sweep, dips into an aperture and scoops out material from that aperture. Other PCB production problems amplified by increased blade pressures are "screen stretch" and contamination of the mask bottom with mounting material. "Screen stretch" is caused by friction between the blade and the mask, which stretches the mask in the direction of the sweep. Contamination of the bottom of the mask with mounting material results in mounting material being applied outside the desired mask pattern and requires cleaning of the mask.
Slow blade movement across the mask may somewhat increase the resolution and definition of mounting material deposition, however, it increases the time to process each circuit board and hence reduces manufacturing rates and increases costs.
The use of an angular sweep to spread mounting material across a mask can somewhat increase the fidelity of mounting material application. One application of this method involves sweeping a blade, which is set at an angle to the sweep direction, down the length of the mask so that the blade passes over the mask apertures, which generally are placed on X and Y axes, at oblique angles. This method of applying mounting material, however, is impractical for the mass production of PCBs due to spacial constraints. To prevent shifting of the mask in a typical current practice, a four-sided frame of minimal size holds the mask tightly in place by securing it on all four sides. As such, when a blade makes an angular sweep, limitations on available space result in solder paste not being applied to two corners of the mask surface.
Accordingly, one objective of the present invention is to provide an apparatus and a method that increases the fidelity of mounting material application in circuit board manufacturing. A particular objective is to more readily attain the mass production of PCBs having a relatively high density of components than is available with conventional production methods. Another object is to reduce wear of both the mask and the squeegee-like blade. Other objects are to allow for the use of a low weight blade and carriage assembly, and to reduce "screen stretch." A further object is to increase the production rate of circuit boards by increasing the allowable sweep rate of the blade across the mask. An additional object is to reduce scavenging of mounting material by the blade.
The invention accordingly comprises the several steps and the relation of one or more such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangements of parts adapted to effect such steps, all as further exemplified in the following detailed disclosure, and the scope of the invention is indicated in the claims.